Mastocytosis is a rare disease with heterogeneous clinical manifestations and few effective therapies. Programmed death-1 (PD-1) and its ligands (PD-L1 and PD-L2) protect tissues from immune-mediated damage and permit tumors to evade immune destruction. Therapeutic antibodies against PD-1 and PD-L1 are effective in the treatment of a variety of neoplasms. In the present study,we sought to systematically analyze expression of PD-1 and PD-L1 in a large number of patients with mastocytosis using immunohistochemistry and multiplex fluorescence staining. PD-L1 showed membrane staining of neoplastic mast cells (MCs) in 77% of systemic mastocytosis (SM) cases including 3 of 3 patients with MC leukemia, 2 of 2 with aggressive SM, 1 of 2 with smoldering SM, 3 of 4 with indolent SM, and 9 of 12 with SM with an associated hematologic neoplasm (SM component only). Ninety-two percent (23 of 25) of cutaneous mastocytosis (CM) cases and 1 of 2 with myelomastocytic leukemia expressed PD-L1, with no expression found in 15 healthy/reactive marrows, 18 myelodysplastic syndromes (MDSs), 16 myeloproliferative neoplasms (MPNs), 5 MDS/MPNs, and 3 monoclonal MC activation syndromes. Variable PD-L1 expression was observed between and within samples, with PD-L1 staining of MCs ranging from 10% to 100% (mean, 50%). PD-1 dimly stained 4 of 27 CM cases (15%), with no expression in SM or other neoplasms tested; PD-1 staining of MCs ranged from 20% to 50% (mean, 27%). These results provide support for the expression of PD-L1 in SMand CM, and PD-1 expression in CM. These data support the exploration of agents with anti-PD-L1 activity in patients with advanced mastocytosis.
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